Analogy of Bound and Light. 191 



case of light ? Do the three curves all represent light ? May we 

 not assume that one of them, as C D, represents heat ; another, 

 as C B, light ; and the other, A B, actinism ? And if this ap- 

 propriation of the three curves be allowed, what part of C D 

 represents sensible heat, what part of C B sensible light, and what 

 part of A B sensible actinism ? It seems reasonable to suppose 

 that heat will, by means of delicate instruments, be sensible from 

 C ; but the moderate variation in the lengths of the waves of 

 sensible light renders it probable that the portion of C B which 

 represents sensible light will commence at some distance from 

 C ; for the length of a wave is represented by the tangent of the 

 angle subtended at A by the ordinate which represents the velo- 

 city of its propagation. As to actinism, but little is known at 

 present to guide us in determining where the portion of A B 

 lies which represents its sensible effects. It seems almost cer- 

 tain, however, on account of the extreme smallness of the ordi- 

 nates of A B, which represent the velocity of its transmission, 

 that only the part of A B which lies towards B will represent 

 effects of actinism which come from the sun. Those represented 

 by the part of A B which lies towards A travel too slowly to 

 reach us, and must be, besides, extremely feeble. Judging then 

 on these principles, we conclude that, roughly speaking, the 

 part of C D beginning from C represents sensible heat ; the 

 middle of C B sensible light ; and the part of A B near to B sen- 

 sible actinism. And this being so, it would seem that heat, if 

 moderate, ought to be found at the red end of the spectrum, and 

 actinism at the blue end. 



25. If the process by which the temperature of a body is 

 raised be such as to give rise to each of the three sets of waves 

 which correspond to a set of values of k, and if, further, the 

 process be such us to produce, as it goes on, larger and larger 

 values of "jfc it is easy to see, after what was said in the last 

 article, that the heat is sensible from the beginning ; but not so 

 the light, nor the actinism. But as the temperature rises, and 

 k takes larger values, we approach the middle of C B, so that 

 by-and-by the light will become sensible; and this will first 

 happen to the red light. As the process goes on, the other 

 colours will be added ; and the body, from appearing of a red 

 heat, will approach to white heat ; and ultimately the actinism 

 will have become sensible also. There is nothing forced in this 

 explaaation. It follows naturally from the principles previously 

 laid down, and the general results obtained : and thus the con- 

 nexion between light, heat, and actinism is shown to be essentially 

 due to the nature of the elastic medium in which they take place, 

 and to follow necessarily from the hypothesis of finite intervals 

 laid down at the commencement of these investigations. 



